1. Field of the Invention
The present invention relates to an optical low-pass filter for use in an optical system such as a single-tube color video camera or a color video camera employing a solid-state image sensor having a color separation filter.
2. Description of the Prior Art
Generally in a video camera or the like employing a solid-state image pickup element, a subject image focused on the image pickup element is spatially and discontinuously sampled to obtain output image data. In the above case, when the optical data of the subject has spatial frequency components which are higher than the spatial sampling frequency of the image pickup element, the output image data generated from the solid-state image pickup element contains data of a configuration or phantom data which is not included in the subject data.
In other words, such frequency components higher than the Nyquist frequency which cannot be picked up by the solid-state image pickup element are output in the form of phantom data such as aliasing components, Moire, and phantom color.
For the above reason, in an image pickup system, there has been employed a conventional practice of limiting such high spatial frequency components of a subject to be input to the solid-state image pickup element by arranging an optical low-pass filter in the image pickup system. For the above-mentioned purpose, the optical low-pass filter has been made utilizing a crystal plate having a birefringence characteristic or recently utilizing a phase grating. However, any conventional optical low-pass filter has been intended for obtaining a low-pass effect generally equal at every wavelength in the entire components.
Considering the case that the entire transmission wavelength band of a color separation filter used in a recent single plate type image pickup system is separated regarding to every wavelength band of three color components (primaries) of R, G, and B, the following technique attempt has been put into practice taking advantage of the fact that the spatial distribution density, i.e., the spatial frequency of each color separation filter is different at every transmission wavelength band. The technique is that a wavelength band having a distribution of higher spatial frequencies is subjected to sampling at a relatively higher sampling frequency and a wavelength band having a distribution of lower spatial frequencies is subjected to sampling at a relatively lower sampling frequency to obtain appropriate image data.
In order to satisfy the above-mentioned requirements, the optical low-pass filter employed in the optical system is required to have different cutoff frequency characteristics in respect of different wavelength bands. A basic structure for attaining the above-mentioned purpose has been described in the Japanese Patent Laid-Open Publication No. Hei-2-113302. In this conventional wavelength-selective phase-grating optical low-pass filter, a phase difference is constructed in a form of a triangular wave form at a boundary face between two materials which have different refractive index distributions and an identical refractive index at an intended wavelength. In recent years, however, since the size of the image pickup element is attempted to be miniaturized, therefore the cutoff spatial frequency becomes higher. In the conventional structure having a phase difference of a triangular wave form, when in attempt of obtaining a low-pass effect at a higher spatial frequency, it was found as the result of the analysis that there is no solution having an appropriate format. According to the study of the analysis of the phase difference form, it was found it necessary that the materials to be used therefor should satisfy a specific condition in constructing the improved optical low-pass filter.